This invention relates generally to thin plastic shells or skins and more particularly to materials for and methods for manufacturing such thin plastic shells by casting thermoplastic particles against the casting surface of a heated mold so as to melt flow and later cool the material into a thin layer part.
U.S. Pat. No. 4,923,657 discloses a powder casting process in which pigmented plastic particles of thermoplastic material are deposited on a heated surf ace to form a thin plastic shell or skin. While suitable f or its intended purpose, the process disclosed in the ""657 patent uses thermoplastic particles having dimensions in the range of 0.002xe2x80x3 to 0.016xe2x80x3. The particles disclosed in the ""657 patent disclosure were virgin polyvinyl chloride resin; plasticizers; stabilizers and pigments blended together in a high intensity mixer. A typical high intensity mixer is nothing more than a heavy duty household type mixing blender. The blender operates at a speed that will cause frictional heating of the resin. In such processes the virgin PVC resin started with a particle having a diameter between 120 to 140 microns. The PVC particle is malleable and has microscopic cracks or fissures that are observable on the surface of the particles. In fact, because the particles are made during the polymerization process, they are found to be formed of ever smaller round particles that are full of passages into which the plasticizers and additives can wick during high intensity blending. Initially, during blending in the high intensity mixer, approximately 50% of the blending plasticizer is added to the mixer. After high intensity mixing, the PVC resin particle softens and swells to a diameter of 180 to 355 microns. In a typical high intensity mixing process the temperature of the material increases from ambient to approximately 180xc2x0 F. At this temperature the PVC particles can absorb higher concentrations of plasticizers and additives. Since pigments and stabilizers are the most difficult constituents to fully disperse into the resin particle, they are usually the last to be added during a typical compounding cycle. Once they are added the compounding temperature is raised to the peak temperature below which the PVC resin particle will not melt so as to form particle agglomerates. The maximum mix temperature is dependent upon the molecular weight of the PVC and the type of plasticizer that is used. But in all cases, the characteristic of particles formed by high intensity mixer compounding the resin particle continues to soften and grow in size as the plasticizers and additives are absorbed or diffused into the resin particle. After the peak temperature is reached (above which melting occurs) the blended material is transferred to a cooling vessel that is equipped with a large blade that rotates at a low speed, for example, 20 RPM, that stirs the compounded material over the cooling surfaces of the vessel without generating any additional frictional heat. As the compounded material cools, the resin particles being to contract so as to hold the plasticizers and additives within the resin particles. Such particles generally are heat sensitive in use and under high temperature ambient conditions, e.g., plant temperatures greater than 80xc2x0 F., the material softens and can become sticky making it difficult to use in roto-casting and other powder casting processes. The softened material becomes tacky and this in turn can lead to manufacturing problems including bridging between the particles, the presence of holes in skins cast from the material and difficulty in controlling the total weight of shells manufactured from such material. In some cases it has been necessary to refrigerate the material when ambient temperatures are elevated to avoid such problems.
Other problems arise since the individual particles formed by the high intensity mixer process have a roughened surface with fissures and cracks therein. Such materials often do not flow into tight corners found in many powder casting mold configurations. Other thermoplastic materials used in powder casting or slush molding of thin shell thermoplastic parts include material that is cryogenically ground. Such material can have a wide range of shapes and is also characterized by outer surface configurations that include cracks and fissures that are apparent to the eye or under magnifications less than 10xc3x97.
Use of such irregularly shaped particles in processing that includes feeding the particles to a point of use and casting the particles on a heated mold to melt flow the particles and cool them to form thin plastic shells presents several other problems. In the case of slush molding, the particles are retained in a powder box that is rotated to direct an excess charge of material into a mold cavity. Such thermoplastic, irregular particles do not flow smoothly from all corners of a powder box. Furthermore, such particles do not smoothly flow into all parts of complex shaped molds of the type having tight return passages and very small mold surface features that simulate features such as leather grains, stitching or the like. As a result, it is necessary to vibrate the powder box and molds during the various processing steps so that the particles will flow against heated mold surfaces so as to melt and form a skin or shell shape corresponding to the shape of the heated mold surface. The irregular shape also produces an uneven build-up of material on the heated mold surface such that the particles do not melt and flow uniformly against the heated mold surface. As a consequence, the resultant cast part can have irregular backside build-up when the part is cooled and extracted from the mold surface. Such irregularities on the backside require that the nominal thickness of the part be larger than required for a given application which in turn takes more material than in the case of a part that has a uniform shape on its backside.
Further, it has been found that irregularly shaped particles define an extended surface area that tends to collect moisture so that the particles do not flow smoothly onto a casting surface from conventional powder box apparatus. This problem is especially pronounced in the case of hydrophilic thermostatic material such as thermoplastic urethane material (TPU). Additionally, moisture build-up on such irregularly shaped particles can cause variations in the thermal load on the casting surface so that the temperature of the casting surfaces require continual adjustment making it difficult to control the quality of the finished product. Additionally, moisture content in excess of 0.01% will create porosity during the casting sequence because the excess water will flash-vaporize when it comes in contact with the heated tool surface.
In the past, TPU has also been used in roto-casting thin plastic parts. In such case the TPU material can be manufactured in a clear flake form that is pigmented and then cryogenically ground into a powder form. The powder is very fine and has an irregular, very coarse outer surface with cracks and fissures that define a high surface area that will hold moisture. Such moisture content will vary in the irregularly shaped materials causing the TPU particles to collect in hoppers and other handling material prior to use in a melt molding process in which the thermoplastic material is placed on the heated surface of a shaping mold. In the case of roto-casting the material is a fixed weight charge corresponding more or less to the weight of the finished product. In such processes the charge is selected to correspond to the weight of the finished part and the charge is continuously centrifuged against the mold surface to be melted and flowed to form the thin walled part. Examples of such roto-casting processes and methods are set forth in U.S. Pat. Nos. 4,167,382 and 4,767,299. In the past, irregularly shaped particles used in roto-casting were cryogenically ground from thermoplastic materials such as TPU or were formed by polymerizing polyvinyl chloride.
Pigment microspheres are also known. They can be added to resin particles in a hopper and mixed in the extruder of an injection molding machine to impart color to the resin particles. Such pigment microspheres are comprised primarily of pigment material in a binder of clear polyethylene. In such cases the pigment comprises 50% of the total weight of each of the pigment microspheres. Such pigment particles are added to polymeric material such as PVC and alloys of PVC and mixed therewith in mixers prior to injection into injection molding equipment. Typically, such pigment microspheres have been sold in a size range of 0.040 to 0.080 inches. They are suitable for use with virtually all thermoplastics used for injection molding a profile extrusions including polyvinyl chloride, polyethylene, polypropylene, nylon, ABS, polyesters, elastomers and the like and, while suitable for their intended purpose of adding coloring to injection molded parts, have not been suggested for use in various roto-casting or slush molding processes and have not been suitable for such purpose since the microsphere has been comprised of mostly pigment material in a small amount of binder that will not provide desired melting and flow characteristics if used in powder casting processes such as roto-casting or slush molding.
Additionally, it is known to form thermoplastic pellets having a spheroidal shape that are added to other materials in an extruder upstream of injection molding apparatus. Since such particles are mixed and melted with other materials (such as pigment particles) in the extruder they come in a size range of 0.030xe2x80x3 and greater. While suitable for use in charging an extruder for use in an injection molding process, such sized particles have not been found suitable for use in powder casting processes such as roto-casting or slush molding since they are too large for molds having tight returns and small dimensioned mold features.
In accordance with one aspect of the invention, a thermoplastic microsphere is provided for use in roto-casting or slush molding that solves the problem of moisture retention and poor flowability of materials for casting thin plastic shells on a heated mold surface. The thermoplastic microsphere is formed of blended thermoplastic resin and pigment that constitute less than 5% by weight of the blend; the microsphere is spheroidal and has an outer diameter in the range of 0.007xe2x80x3 to 0.040xe2x80x3 and has a surface that appears smooth up to magnifications of 80xc3x97.
One embodiment of the process of the present invention includes providing a predetermined charge of material made up of microspheres having an outer diameter in the range of 0.007xe2x80x3 to 0.040xe2x80x3 and processing thin shells in the range of from 0.020xe2x80x3 to 0.050xe2x80x3 from such microspheres by either roto-casting including placing a charge corresponding to the weight of the finished product in a closed hollow heated mold and centrifugally and melt flowing the material into a shape corresponding to the heated mold surface; thereafter cooling the centrifugally molded part and removing it from the closed hollow heated mold or providing such material and slush molding a charge of material larger than the weight of the part to be cast in a powder box; joining the powder box to a mold having a cavity; rotating the powder box to fill the mold cavity with a charge of material that imposes a static head on the material that rests on the heated casting surface of the mold cavity; melting the material resting on the heated casting surface and flowing it against the heated casting surface; dumping excess material from the mold cavity; cooling the thin build-up of material on the casting surface and thereafter removing the finished part from the mold cavity.
In accordance with another aspect of the invention, the thermoplastic microsphere is formed in a continuous process that includes extruding thermoplastic urethane material and flaking the TPU into flakes xe2x85x9xe2x80x3xc3x97xe2x85x9cxe2x80x3; adding pigment; melting the resultant mixture and directing it in the molten state and cutting it into particles that are cooled in a water chamber to cause the particles to be shaped into microspheres having an outer diameter in the range of 0.007xe2x80x3 to 0.040xe2x80x3 and an outer surface that appears smooth up to a magnification of 80xc3x97.
Still another aspect of the present invention is to use polyvinyl chloride material including pure PVC and alloys with PVC as the starter material and mixing it with pigment; melting the resultant mixture and directing it in the molten state and cutting it into particles that are cooled in a water chamber to cause the particles to be shaped into microspheres having an outer diameter in the range of 0.007xe2x80x3 to 0.040xe2x80x3 and an outer surface that appears smooth up to a magnification of 80xc3x97.
Still another aspect of the present invention is to use TPU as the starter material wherein the TPU is mixed with pigment; melting the resultant mixture and directing it in the molten state and cutting it into particles that are cooled in a water chamber to cause the particles to be shaped into microspheres having an outer diameter in the range of 0.007xe2x80x3 to 0.040xe2x80x3 and an outer surface that appears smooth up to a magnification of 80xc3x97.
Still another aspect of the present invention is to use a thermoplastic material starting material selected from the group comprising polyurethane; polypropylene; polyethylene; polystyrene; epoxies or alloys of such materials; mixing such thermoplastic material with suitable plasticizers (if required) and additives; melt extruding the resultant mixture through an extrusion die having a diameter in the range of 0.007xe2x80x3 to 0.040xe2x80x3 and directing the extrusion stream as molten material through a cutter for forming particles and then cooling the particles in a water chamber so as to form microspheres having diameters in the range of 0.007xe2x80x3 to 0.040xe2x80x3.
A further feature of the present invention is to use any of the aforesaid starting materials with resins having a low molecular weight. For example, in the case of PVC from 50,000 to 150,000 M.W.
Another advantage of the present invention is that lower molecular weight resins in the microspheres will reduce the process temperatures and will decrease cycle time.
Still another advantage of the present invention is that such formation of such microspheres will lock the plasticizer component into the resin matrix material such that the resultant cast part can be formed with a uniform thickness without backside irregularities so as to improve shell weight control while reducing powder tackiness at higher summer time ambient conditions. Still another advantage of the invention is that it reduces the need for vibratory devices to maintain powder flow and as a consequence there is less tool wear which in turn increases tool life.
Still another advantage of the present invention is to make it easier to clean powder boxes and other equipment and thereby enable processing equipment to be quickly changed from one color to another color, e.g., quicker color changeover.